1. Field of the Invention
The invention relates to a novel method and apparatus for determining the level of material in a silo or the like. More specifically, the invention relates to such a method and apparatus which utilizes mathematical autocorrelation techniques.
2. Description of Prior Art
Methods and apparatus for determining levels of material in silos are known in the art. In a general overview, the prior art apparatus will use ultrasound waves, capacitance probes, mechanical approaches or approaches wherein EM waves are deflected from the top surface of the material, and the time travel of the EM wave is measured to determine the level of the material.
In the ultrasound approach, the echo of a pulse is detected to measure distance. The problem with this approach is that the velocity of longitudinal sound waves changes with temperature, pressure, humidity, and dust-laiden air. In addition, the pulse signal is dispersed and scattered by dust and particles in a tank or silo so that operation is unreliable or impossible during a fill cycle. Further, the echo can be lost due to low density absorbent products, foaming liquids and scattering of irregular and sloping product surfaces. Another difficulty is that the long wavelength of ultrasound makes it impossible to focus a signal. In deep narrow silos, the signal cannot be targeted on the product. Further, spurious indirect echoes off tank walls will mask returns from product surfaces making the method even more unreliable. Plant noise and vibration will also interfere with ultrasonic receivers.
In the capacitance probe approach, the level of material in a silo or the like is indicated by the capacitance of wires or probes installed in the tank. As the material has a different dielectric constant than air, the capacitance of the wire or probe will alter with the change in level of the material in the tank.
For applications in products with large variations in dielectric constants, this technique is unworkable. As most dry bulk powders and granulates have a low dielectric constant whereas water has a high value, variations, even small, of moisture content in bulk product causes very large calibration errors.
In addition, the sensing wire must be fixed relative to the tank wall to maintain constant capacitor geometry. In large bulk material silos, lateral slipping of the product will break a tethered sensor wire so that the system becomes completely unusable.
In a mechanical approach, also known as a "yo-yo" approach, a weight on a string is dropped into the silo and the level is determined by measuring the length of string payed out prior to loss of tension in the string on the weight impacting on the top level of the material. Such mechanical devices are prone to failure in harsh, dusty environments and they provide a very slow-acting sampler. In addition, the mechanical devices cannot be used when the silo is being filled as the weight will get trapped.
The EM deflection approach uses either conventional pulse-echo EM waves or CW waves. Such an approach will work only in large diameter tanks with a flat, smooth product surface. For a practical antenna size (diameter--one foot maximum), and for the wavelengths used in conventional such systems, the signal cannot be focussed to target only on the top surface of the material in the tank.
For dry bulk products, the reflecting surface is invariably sloping. Thus, the pulses or continuous waves will be reflected at an angle to the longitudinal axis of the silo and will thence be deflected off the silo or tank walls. Such indirect signals off the walls will also enter the receiver due to the wide beam width of the antenna causing serious errors.
Specific examples of prior art devices are illustrated in U.S. Pat. No. 4,596,144, Panton et al, Jun. 24, 1986, U.S. Pat. No. 4,661,817, Bekkadal et al, Apr. 28, 1987, U.S. Pat. No. 4,700,569, Michalski et al, Oct. 20, 1987, and U.S. Pat. No. 4,807,471, Cournane et al, Feb. 28, 1989.
In U.S. Pat. No. 4,596,144, the top level of material in a silo is determined by directing at least one transducer 2 ,of a system with a plurality of transducers into the silo. The transducers are then activated one at a time by acoustic pulses, and the reflected signals are processed by a receiver having three channels, each with different characteristics (FIG. 2). The outputs of the three channels are summed to provide a composite response signal. The composite signal is then digitized and fed to a RAM 26 of CPU 30. The shots from the different transducers are then processed, as per the flow chart of FIG. 12 in the Patent, to determine the level of the material in the silo.
In the '817 patent, part of a generated FM signal is transmitted from a transducer into a silo and the remainder is used as a reference signal. The reflected signal and the reference signal are then used to obtain a value for the input reflection co-efficient of the antenna. This value is subjected to a Fourier transformation process at a predetermined set of measuring frequencies, and the distance from the antenna to the surface is calculated from the transformation process values.
If the '569 patent, a series of acoustic pulses are transmitted from a transducer into a silo. The echo pulses are converted to electrical envelope signals representative of the envelope curves of the echo pulses. The echo signals are then transmitted to an evaluation station where they are evaluated to determine the distance from the antenna to the top of the material in the silo. The evaluation station is a computer and the envelope signals are digitized in an ADC before being transmitted to the microprocessor.
In the '471 patent, an elongated cable extends into the silo or the like and the output of a swept frequency generator is fed to the cable. The swept frequency generator is automatically and continuously swept through a pre-selected frequency range. Peak voltages will occur at frequency intervals, the frequency intervals being a function of the level of the material in the silo or the like. The frequency intervals are detected to determine the level of the material.